In a conventional engine powered vehicle with an automatic transmission, a transmission controller manages the operation of the torque converter. The torque converter has a bypass clutch that manages torque transfer between the impeller and turbine of the torque converter. A bypass clutch may provide three modes of bypass clutch operation, and torque multiplication may occur depending on the amount of slip between the impeller and turbine sides. In open mode, a maximum amount of fluid is carried by the torque converter housing, separating the impeller from the turbine. In a locked mode, a minimum fluid pressure is carried in the torque converter so the pressure does not separate the impeller from the turbine and they become mechanically locked together, by which the fuel economy is preserved. In a slip mode, a limited amount of slip may be employed between the impeller and the turbine, whereby the fluid may provide the target torque ratio for the torque multiplication, in addition to NVH damping, but fuel economy is reduced due to the heat generated as a result of the slip.
In the slip mode, a target amount of slip called micro slip may be controlled by a feed forward control that commands the fluid pressure in the torque converter housing, a closed loop feedback control monitoring the slip, or a combination of both. However, the feed forward control predicts operation of a system based upon the engine torque estimation, but the difficulty in accurately estimating the engine torque value during an event such as tip-in, and the timing delay of a feedback circuit interfere with accurate control of the slip during a micro slip control event. Hybrid vehicles add to the complexity of powertrain control by including the monitoring and control of engine, traction motor, or both as motive sources in the driveline, as well as clutches and additional sensors and actuators.